June 19, 2003
10:00 AM (EDT)

News Release Number: STScI-2003-18

The Secret Lives of Galaxies Unveiled in Deep Survey

June 19, 2003: Two of NASA's Great Observatories, bolstered by the largest
ground-based telescopes around the world, are beginning to
harvest new clues to the origin and evolution of the
universe's largest building blocks, the galaxies. It's a bit
like finding a family scrapbook containing snapshots that
capture the lives of family members from infancy through
adolescence to adulthood. The Hubble Space Telescope has
joined forces with the Chandra X-ray Observatory to survey a
relatively broad swath of sky encompassing tens of thousands
of galaxies stretching far back in time. Called the Great
Observatories Origins Deep Survey (GOODS), astronomers are
studying galaxy formation and evolution over a wide range of
distances and ages.

Q & A: Understanding the Discovery

1.
What has Hubble discovered about galaxy evolution?

In preliminary results soon to be published, Hubble
astronomers report that the sizes of galaxies clearly
increase continuously from when the universe was about 1
billion to 6 billion years old. (Many astronomers believe
that the universe is about 13.7 billion years old.) GOODS
astronomers also find that the star-birth rate rose slightly between the time the universe was about 1 and 1.5 billion
years old, and remained high until about 7 billion years ago,
when it quickly dropped. This is further evidence that major
galaxy building trailed off when the universe was about half
its current age.

2.
What is Chandra's contribution to the survey?

The Chandra observations amounted to a "high-energy core
sample" of the early universe. One of the fascinating
findings in this deepest X-ray image ever taken is the
discovery of seven mysterious black holes that do not
correspond to the galaxies seen in the Hubble image.
Astronomers suspect that these objects are the most distant
black holes ever detected, or the galaxies in which they
reside cannot be seen because they are heavily enshrouded in
dust.

When comparing the Hubble and Chandra fields, astronomers
also found that active black holes in distant, relatively
small galaxies were more rare than expected. This may be due to the effects of early generations of massive stars that
exploded as supernovas, evacuating galactic gas and thus
reducing the supply of gas needed to feed a supermassive
black hole.

3.
Why did astronomers use several observatories to study the same regions of sky?

Astronomers used several observatories for this study because
they want to build a coherent picture of galaxy evolution.
The two orbiting observatories, Chandra and Hubble, analyzed
different wavelengths of light. Chandra studied X-rays from
supermassive black holes; the Hubble telescope's Advanced
Camera for Surveys examined visible light from stars in
galaxies. Another space observatory, the Space Infrared
Telescope Facility (SIRTF), will sample the same regions of
sky in infrared light soon after it is launched in August
2003.